The present invention relates to methods and apparatus for joining connective members, especially conductive members to join electronic and microelectronic circuits.
Electronic circuits, such as microelectronic logic circuits, or other similar electronic circuits, may be fabricated as an integrated unit, which has developed to a highly efficient method of compact circuit manufacture. Ultimately, however, the integrated circuit components or other packaged electronics must be connected to larger circuits in order to be utilized, and be interconnected via lead frames or other connectors with circuits such as input and display apparatus, power supplies and grounds, and complementary circuits. Given the small scale of such circuits, the connection of these circuits also takes place on a relatively small scale. For example, integrated circuit chips are typically less than 0.3 inchesxc3x970.3 inches. These circuits may be interconnected by very small wires, e.g. wires with a one mil diameter, or by small, flat conductive metal ribbons which may be, for example, 1xc3x9710 mils (0.001 inchesxc3x970.01 inches).
Generally, metals for electronic connections may be joined by soldering, i.e., by the melting of an alloy or element with a relatively low melting temperature, where neither base material of the joined members is melted or becomes part of the joint. Welding, in contrast, involves the melting of the base members to be joined, resulting in the formation of a weld nugget consisting of material from both of the elements to be joined, in other words, a fusion or thorough and complete mixing between the two edges of the base metal to be joined.
While heat is often used to join wires or other conductors together, both for solid-state, fusion, and solder/brazing applications, many traditional methods of heating have proved to have drawbacks in microelectronic applications. One method of applying heat to a bonding site has involved heating the bonding head to convey heat to the bonding site. In an alternate method, a heater block may be clamped to a circuit lead frame. However, heat applied to these structures, which are large relative to the area to be bonded, may cause distortion or bending of the lead frame, or damage to other electrical components. If heat is to be used to solder microelectronic connectors, it would be desirable to more precisely localize this heat on the leads or connectors to be soldered, rather than heating an area as large as, for example, an entire lead frame.
As an alternative to soldering using heat alone, ribbon bonders of the prior art have used specialized solid-state bonding methods, e.g., ultrasonic energy, to bond the ribbons to substrates, lead frames, or various electronic components. Ultrasonic energy, a high-frequency vibration, e.g. from 60 KHz to over 100 KHz, is imparted to the parts to be bonded by a bond head. This vibration, and the attendant abrasion of the connector against the terminal pad or lead, in conjunction with heat and mechanical pressure from the bonding head, effects metallurgical atomic diffusion bonding of the connector with the metal of the bonding sites. Modern ultrasonic bonding machines conveniently employ optics and pattern-matching logic systems in order to automate the bonding process for a particular package or circuit being assembled.
Ultrasonic ribbon bonding is primarily employed as a method of connecting integrated circuits, packages or substrates in high frequency or high power applications. Ultrasonic bonding techniques, however, have several drawbacks which motivate a reduced reliance on such techniques. Ultrasonic or thermosonic bonding, i.e. ultrasonic vibration using heat, may find application in bonding flat, rigid structures but is not well suited to bonding less rigid, i.e., flexible or semi-flexible structures. Such structures tend to vibrate in response to the ultrasonic energy causing much of the energy to be lost rather than creating the intended bond. Another drawback of ultrasonic bonding is that it may be used primarily with certain materials, and is generally limited to gold, aluminum and copper. Accordingly, the substrate metal to be bonded is generally gold plated.
Because the ultrasonic energy is, in fact, a vibration, albeit a very high-frequency one, this vibration may cause undesired movement of the parts to be joined during the bonding process. Not only can this lead to dislocation of the parts vis-à-vis each other, but the movement of the parts during the time when bonding is being effected naturally results in a weaker and inconsistent bond. These problems present substantial vibrational stability requirements for the terminals and substrates used in ultrasonic bonding. In light of the limitations of ultrasonic bonding, an alternative method of bonding conductors for microelectronic devices would be desirable.
Resistance welding has enjoyed limited application in microelectronics manufacture. To varying extents, metallic objects resist the flow of electrical current. This resistance will cause heat energy as electric current passes through the metals to be bonded. The higher the amperage and duration of current, the greater the heat energy that will be produced. Metallic objects have thermal properties, a melting point, a specific heat content, thermal conductivity, and more. By using these properties, an environment can be created to produce a molten pool that will harden into a welding nugget. However, the application of resistance welding is limited, and is generally incompatible with low resistance ribbon materials such as copper, silver and gold.
Laser-generated heat has found application in certain part-joining methods. For example, lead frames have been soldered with lasers in applications such as TAB (Tape Automated Bonding), utilized, for example, in U.S. Pat. No. 4,893,742 to Bullock. TAB bonding, however, is subject to a number of limitations, chief among them that dedicated and expensive equipment is necessary for each process step. The interconnecting ribbons or leadframes must be formed ahead of time. Therefore, the specially designed tape carriers for each type of circuit being produced involve long lead time and high cost. Dedicated tooling is required to excise and form TAB leads. xe2x80x9cBumpingxe2x80x9d, i.e., the placement of small metal bumps on the circuit bond sites in order to provide a bonding surface above the circuit""s passivation layer, is required. In addition, the leadframes must be placed and held precisely in position before soldering. Finally, the leads to be bonded with existing apparatus and methods require solder to effect bondingxe2x80x94the main body of the lead and connectors do not reach a melting point, and only the solder is softened. TAB leads are accordingly coated with solder at some point prior to the bonding process. Therefore, considerable advance preparation of the TAB leadframes is required. In general, characterizing the behavior of individual designs and structures is very time-consuming, as is the construction of lead frame tapes for TAB production methods.
It would be desirable to provide laser bonding which could be effected using devices similar to traditional wedge bonding equipment, which may automatically bond individual contact points, without the preparation required for tape-mounted lead frames. It would also be desirable to provide a bonding method that would work on a variety of materials, including low-resistance metals, without the use of solder. In addition, it would be desirable to provide a bonding technique that utilizes a highly localized heating area, without peripheral heating of lead frames or other components adjacent the bond site. Finally, it would be desirable to have a bonding method capable of bonding flexible materials that may tend to vibrate in response to the application of ultrasonic energy.
Difficulties with existing systems of connector fabrication are overcome with a device providing for laser bonding of ribbon connectors, especially conductive connectors used to provide current pathways for the operation of electronic or microelectronic components. In an alternative embodiment, the device is adapted for use with any material which absorbs the particular wavelength of the laser used. For example, the device may be used to bond non-conductive ribbons, i.e. plastic connectors such as may be used in packaging or other applications. In a further alternative embodiment, the invention may also be used to bond conductive connectors of alternative configurations, e.g. round wire. Preferably, a bonding device according to the present invention utilizes automation as developed for traditional wire and ribbon bonding, e.g. pattern-matching automation.
According to one embodiment of the invention, an automated pattern-matching bonder welds one end of an interconnection ribbon, the ribbon being fed from a spool of suitable interconnection ribbon. Subsequently, the bond head of the device moves to the second bond location as it spools out and forms the ribbon into the desired loop shape and then welds the second connection and terminates the ribbon. Alternatively, after formation of a second bond, additional ribbon may be spooled out to form one or more additional loop connectors, each loop terminating at a new weld connection. As an alternative to movement of the bond head, a machine table on which the work piece is mounted may move while the bond head remains stationary. The present invention has potential application for internal device interconnection, i.e., connections internal to an IC package, as well as final board assembly and other microelectronic connections. When bonding is effected according to the present invention, the choices for both ribbon and substrate materials increases, and the dependence on structural rigidity and terminal stability, as required for ultrasonic bonding, decreases or is eliminated.
Microelectronic bonding has enjoyed particularly useful application in the implantable medical device art, for example, as demonstrated in U.S. Pat. No. 5,535,097 to Ruben, et al. and U.S. Pat. No. 5,522,861 to Sikorski, et al., both assigned to the assignee of the instant application and both of which are hereby incorporated by reference. By way of example, the present invention may be used to make electrical connectors between and among the hybrid circuit, battery, capacitors, feedthroughs, and other components of implantable medical devices. In addition, however, it will be appreciated to those skilled in the art that the instant invention may be used in various microelectronic applications. These may include, but are not limited to, semiconductor production and chip utilization, integrated circuit packaging and mounting, and other electrical interconnections in the computer hardware and electronics industries.